Asynchronous transfer mode (ATM) is a mode of fast packet switching which allows systems to operate at a much higher rate than traditional packet switching systems. Features which characterizes ATM communications are: the ability for asynchronous operations between a sender clock and a receiver clock; transmission "cells" of pre-defined sizes; operation at a basic frame rate, with all transmissions being at integer multiples or devisors of the frame rate; and addressing carried out in a fixed size header (that is not by time, frame position or other fixed characteristic). ATM communication is sometimes also referred to as asynchronous time division (ATD) communications.
Attention has turned to the use of ATM for wireless or radio communications. There is, for example, a need for wireless users to have access to wired ATM networks and existing ATM systems such as multi-media applications need a wireless platform providing multi-media support. It is also recognized that systems such as universal mobile telephone systems (UMTS) and wireless local area networks (LANs) cannot meet all future data user needs. Efforts to date have been in the use of ATM in the wireless extension of fixed infrastructure systems, such as LANs and integrated service data network (ISDN).
U.S. patent application Ser. No. 08/641797, filed on 2 May 1996 and assigned to the assignee of the present invention describes a radio unit, a method of communicating between radio units over a communications channel and method of preparing a sequence of ATM cells for transmission over a radio channel and is incorporated herein by reference. In that patent application it is described how a channel can be considered as divided into frames and the start and end of a frame need not be synchronized across the system, provided that each two communicating units operate to the same frame length. Access by a radio unit to a frame location is described as implying access to the corresponding locations of subsequent frames. It is also described that some services may require a fractional number of cells per frame. While such an arrangement has the ability to provide much more efficient channel usage, efficiency may not be achieved if the manner of allocation of the available frame resources is not also efficient.
U.S. patent application Ser. No. 08/641797 describes an etiquette (referred to as "SAMA") based on the use of ATM style packets over the air. An etiquette can be considered to be a set of rules, agreed upon by all participants, that provide a group of individuals the ability to cooperate in an activity without the requirement for mutual communication. A protocol differs from an etiquette in one important aspect; the devices participating in a protocol must be capable of mutual communication.
There is a need for a communications system that provides a simple bandwidth setup mechanism; supports devices with widely varying bit rates; reduces the number of "collisions" and support the development of both ad-hoc and centrally controlled protocols.
A goal in the development of an ATM etiquette is the simplification of procedures for gaining access to the channel. In more traditional systems, this can takes several hundred of milliseconds and consume considerable bandwidth. In SAMA, access to the channel is gained by sending a "probe cell" in the desired frame slot and receiving an acknowledgment (ACK) in the corresponding slot of the next frame. The time taken by this exchange is, of course, a function of frame size, but will typically be less that fifteen milliseconds.
Referring to FIG. 1, if a subscriber unit 10 sends a probe packet to a subscriber unit 14 in a slot that is in use by another pair of subscriber units 11 and 12, a collision occurs. The collision is confined to one data packet and has three possible outcomes: (i) the packet destined for subscriber unit 11 is destroyed and the packet destined for subscriber unit 14 succeeds; (ii) the packet destined for subscriber unit 14 is destroyed and the packet destined for subscriber unit 11 succeeds; (iii) both packets are destroyed.
There is a need for an efficient method of allocating frame locations in a wireless ATM communications system and minimizing collisions.
A preferred embodiment of the invention is now described, by way of example only, with reference to the drawings.
______________________________________ Glossary of Terms ______________________________________ ATM Asynchronous transfer mode; LAN Local area network RSSI Received signal strength indication; SAMA "Simple" ATM Multiple Access; VCI Virtual circuit identifier; VPI Virtual path identifier. ______________________________________